Field of the Invention
The present invention relates to a wireless communication system and more particularly a method for a user equipment to select a codebook index in a wireless communication system and an apparatus supporting the method.
Related Art
3rd generation partnership project (3GPP) long term evolution (LTE) is an improved version of a universal mobile telecommunication system (UMTS) and is introduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink, and uses single carrier-frequency division multiple access (SC-FDMA) in an uplink. The 3GPP LTE employs multiple input multiple output (MIMO) having up to four antennas. The 3GPP LTE adopts MIMO (multiple input multiple output) having maximum four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-advanced (LTE-A) that is an evolution of the 3GPP LTE.
Recently, Multiple Input Multiple Output (MIMO) systems are getting much attention in an effort to maximize performance of wireless communication systems and communication capacity. Evolved from the conventional method of using a single transmit antenna and a single receive antenna, MIMO technology may improve efficiency of transmitting and receiving data by adopting multiple transmit antennas and multiple receive antennas. A MIMO system is also called a multiple antenna system. MIMO technology relies on integration of data segments received from multiple antennas to get a single, whole message instead of relying on a single antenna path. As a result, data transmission speed can be increased within a specific range, or system operation scope can be increased with respect to a specific data transmission speed.
Elements of MIMO technology include transmit diversity, spatial multiplexing, and beamforming. Transmit diversity refers to the technology for improving transmission reliability by transmitting the same data through multiple transmit antennas. Spatial multiplexing refers to the technology for transmitting high speed data without increasing system bandwidth by transmitting different data simultaneously through multiple transmit antennas. Beamforming is used to increase SINR (Signal to Interference plus Noise Ratio) of a signal by applying channel-dependent weights in a multiple antenna system. At this time, weights may be expressed in terms of a weight vector or weight matrix, which is called a precoding vector or precoding matrix respectively.
Spatial multiplexing may be further divided into spatial multiplexing for a single user and spatial multiplexing for multiple users. Spatial multiplexing for a single user is also called SU-MIMO (Single User MIMO) while spatial multiplexing for multiple users is called SDMA (Spatial Division Multiple Access) or MU-MIMO (Multi-User MIMO). Capacity of a MIMO channel increases in proportion to the number of antennas. A MIMO channel may be decomposed into independent channels. Given that the number of transmit antennas is Nt, and the number of receive antennas is Nr, the number of independent channels Ni becomes Ni≤min{Nt, Nr}. Each independent channel may be regarded as a spatial layer. A rank represents the number of non-zero eigenvalues of the MIMO channel matrix and may be defined as the number of spatial streams that may be multiplexed.
MIMO technology uses a codebook-based precoding technique. The codebook-based precoding technique has been devised to transmit a precoding matrix indicator (PMI) by selecting a precoding matrix most similar to the MIMO channel among predetermined precoding matrices, which may be used to reduce overhead due to recursive data. A codebook comprises a codebook set which may represent spatial channels. To increase transmission rate of data, the number of antennas has to be increased, but the codebook has to be prepared with a larger number of codebook sets in proportion to the number of antennas. Because of the increase of codebook sets in proportion to the number of antennas, not only the overhead due to recursive data is increased but also designing a codebook becomes difficult.